[18F] fluoride is produced by irradiation of water, containing H218O, with protons resulting in the reaction 18O(p,n)18F. Only a minor fraction of the [18O] is converted. The [18F] isotope is then separated from the water and processed for production of a radiopharmaceutical agent.
In the current practice, fluoride recovery is based on the use of an anion-exchange resin. The recovery is carried out in two steps, extraction and elution: first the anions (not only fluoride) are separated from the enriched [18O] water and trapped on the said resin. The anions, including [18F] fluoride, are then eluted into a mixture containing water, organic solvents, a phase transfer agent or activating agent or phase transfer catalyst, such as for example the complex potassium carbonate-Kryptofix 222 (K2CO3—K222) or a tetrabutyl-ammonium salt. The [18F] fluoride radiochemical recovery yield is very effective, usually exceeding 99%.
The most usual labeling method, known as nucleophilic substitution, requires anhydrous or very low water content solutions. Thus, an evaporation step (or drying step) is still necessary after recovery to remove the excess water. It usually consists in multiple azeotropic evaporations of acetonitrile or of low boiling temperature organic solvent. Such evaporations require several minutes.
The current trend in the automation of the preparation of radiopharmaceuticals for medical imaging is to develop “Lab-on-chip” devices. The aforementioned evaporation step is very difficult to implement within such a “Lab-on-chip” device.